add multiple key support to signapk
add multiple key support to signapk

Support signing .apks (but not OTA packages) with multiple keys.

Bug: 7350459
Change-Id: I794e1da0555e2bb9247a59c756656d4ca7ee04cf

xorware/android_build,xorware/android_build,xorware/android_build,xorware/android_build,xorware/android_build